If you've ever wondered why you never get a fly in your coffee, a Harvard scientist has finally come up with the answer.

Caffeine and related compounds are natural insecticides that plants use toward off damaging pests, reports James A. Nathanson an assistant professor of neurology at the Medical School.

While people for centuries have used coffee, ten and other caffeinated plants is stimulants, the natural function of caffeine in plants has, until now remained a mystery, Nathanson said in a report published in Science magazine last week.

Moreover, the results may eventually lead to the development of caffeine--based insecticides which would be relatively harmless to animals and humans for spraying on food crops.

Scientists have known for years that plants, have naturally developed chemicals to protect themselves, Nathanson said, but only recently was it suspected that caffeine served this important function.

While many scientists labeled the results suprising, some downplayed the over all significance.

"I find it considerably interesting and even amusing," said Bussey Professor of Biology Carroll M. Williams. "Caffeine may be an insecticide, but it is not a very powerful agent."

The study also determined that caffeine combined with other insecticides increases their killing power. In one test, a small dose of caffine increased a known pesticide's potency by ten times.

The potency of the combination, the report stated, was far greater than that of the sum of the parts.

While caffeine may hold potential for use as an insecticide, a National Coffee Association official said that it is still too early to determine if the new data will have major financial implications.

"It's all good and well, but the really important thing is that the research didn't show caffeine poses a hazard to the public," he added.

Nonetheless, any use of caffeine as an insecticide is still several years off in the future. It might take as many as five years for the completetion of large scale testing to determine if caffeine--based insecticides are practical or useful.

The behavior and growth of numerous insects and their larvae were considerably disturbed when treated with caffiene and related compounds, Nathanson reported.

At concentrated doses, the substance killed insects within hours or a few days, the report said, and it distorted behavior, depressed food consumption and inhibited reproduction in tobacco hornworms, milkweed bugs, butterfly larvae, and mosquito larvae.

For example, mosquito larvae became so uncoordinated with exposure to caffeine that they could not swim to the water's surface and drowned.

Caffeine appears to produce the destructive effect by supressing certain enzymes in the insects' nervous system.